The SimpleActions plugin now contains an action to highlight all methods of an interface/class that are implemented/overridden by the current class.
For example, if your class Foo implements an interface IntA, choosing the action 'Highlight Implementations in Class' from the Search menu or pressing Ctrl-Alt-Shift-F7 while the caret is over a reference to IntA, will highlight all methods of class Foo that are part of the interface IntA. Slightly different highlight colors are used for 'direct' implementation/override vs. method defined on interface/class further down the hierarchy. F3 for 'goto next' is supported, too (thanks Bas!).
I have uploaded the new version SimpleActions 1.1.0 to the plugin repository, in case anybody besides me is interested in this functionality.
PS. Compiled against IDEA 5.0, tested against IDEA 5.1, running in Demetra currently fails due to OpenAPI change.